On Mon, Sep 18, 2023 at 12:33 AM Richard Biener via Gcc <gcc@gcc.gnu.org> wrote: > > On Sat, Sep 16, 2023 at 10:38 AM Martin Uecker via Gcc <gcc@gcc.gnu.org> > wrote: > > > > > > > > (moved to gcc@) > > > > > On Fri, Sep 15, 2023 at 08:18:28AM -0700, Andrew Pinski wrote: > > > > On Fri, Sep 15, 2023 at 8:12 AM Qing Zhao <qing.z...@oracle.com> wrote: > > > > > > > > > > > > > > > > > > > > > On Sep 15, 2023, at 3:43 AM, Xi Ruoyao <xry...@xry111.site> wrote: > > > > > > > > > > > > On Thu, 2023-09-14 at 21:41 +0000, Qing Zhao wrote: > > > > > >>>> CLANG already provided -fsanitize=unsigned-integer-overflow. GCC > > > > > >>>> might need to do the same. > > > > > >>> > > > > > >>> NO. There is no such thing as unsigned integer overflow. That > > > > > >>> option > > > > > >>> is badly designed and the GCC community has rejected a few times > > > > > >>> now > > > > > >>> having that sanitizer before. It is bad form to have a sanitizer > > > > > >>> for > > > > > >>> well defined code. > > > > > >> > > > > > >> Even though unsigned integer overflow is well defined, it might be > > > > > >> unintentional, shall we warn user about this? > > > > > > > > > > > > *Everything* could be unintentional and should be warned then. GCC > > > > > > is a > > > > > > compiler, not an advanced AI educating the programmers. > > > > > > > > > > Well, you are right in some sense. -:) > > > > > > > > > > However, overflow is one important source for security flaws, it’s > > > > > important for compilers to detect > > > > > overflows in the programs in general. > > > > > > > > Except it is NOT an overflow. Rather it is wrapping. That is a big > > > > point here. unsigned wraps and does NOT overflow. Yes there is a major > > > > difference. > > > > > > Right, yes. I will try to pick my language very carefully. :) > > > > > > The practical problem I am trying to solve in the 30 million lines of > > > Linux kernel code is that of catching arithmetic wrap-around. The > > > problem is one of evolving the code -- I can't just drop -fwrapv and > > > -fwrapv-pointer because it's not possible to fix all the cases at once. > > > (And we really don't want to reintroduce undefined behavior.) > > > > > > So, for signed, pointer, and unsigned types, we need: > > > > > > a) No arithmetic UB -- everything needs to have deterministic behavior. > > > The current solution here is "-fno-strict-overflow", which eliminates > > > the UB and makes sure everything wraps. > > > > > > b) A way to run-time warn/trap on overflow/underflow/wrap-around. This > > > would work with -fsanitize=[signed-integer|pointer]-overflow except > > > due to "a)" we always wrap. And there isn't currently coverage like > > > this for unsigned (in GCC). > > > > > > Our problem is that the kernel is filled with a mix of places where there > > > is intended wrap-around and unintended wrap-around. We can chip away at > > > fixing the intended wrap-around that we can find with static analyzers, > > > etc, but at the end of the day there is a long tail of finding the places > > > where intended wrap-around is hiding. But when the refactoring is > > > sufficiently completely, we can move the wrap-around warning to a trap, > > > and the kernel will not longer have this class of security flaw. > > > > > > As a real-world example, here is a bug where a u8 wraps around causing > > > an under-allocation that allowed for a heap overwrite: > > > > > > https://git.kernel.org/linus/6311071a0562 > > > https://elixir.bootlin.com/linux/v6.5/source/net/wireless/nl80211.c#L5422 > > > > > > If there were more than 255 elements in a linked list, the allocation > > > would be too small, and the second loop would write past the end of the > > > allocation. This is a pretty classic allocation underflow and linear > > > heap write overflow security flaw. (And it would be trivially stopped by > > > trapping on the u8 wrap around.) > > > > > > So, I want to be able to catch that at run-time. But we also have code > > > doing things like "if (ulong + offset < ulong) { ... }": > > > > > > https://elixir.bootlin.com/linux/v6.5/source/drivers/crypto/axis/artpec6_crypto.c#L1187 > > > > > > This is easy for a static analyzer to find and we can replace it with a > > > non-wrapping test (e.g. __builtin_add_overflow()), but we'll not find > > > them all immediately, especially for the signed and pointer cases. > > > > > > So, I need to retain the "everything wraps" behavior while still being > > > able to detect when it happens. > > > > > > Hi Kees, > > > > I have a couple of questions: > > > > Currently, my thinking was that you would use signed integers > > if you want the usual integer arithmetic rules we know from > > elementary school and if you overflow this is clearly a bug > > you can diagnose with UBsan. > > > > There are people who think that signed overflow should be > > defined to wrap, but I think this would be a severe > > mistake because then code would start to rely on it, which > > makes it then difficult to differentiate between bugs and > > intended uses (e.g. the unfortunate situation you have > > with the kernel). > > > > I assume you want to combine UBSan plus wrapping for > > production use? Or only for testing? Or in other words: > > why would testing UBSan and production with wrapping > > not be sufficient to find and fix all bugs? > > > > Wrapping would not be correct because it may lead to > > logic errors or use-after-free etc. I assume it is still > > preferred because it more deterministic than whatever comes > > out of the optimizer assuming that overflow has UB. Is this > > the reasoning applied here? > > > > > > For unsigned the intended use case is modulo arithmetic > > where wrapping is the correct behavior. At least, this > > is what I thought so far.. This seems also to be the > > position of the overall GCC community rejecting > > -fsanitize=unsigned-integer-overflow. > > > > But then there are also people like Richard Seacord that > > have the position that one should use "unsigned" for > > every quantity which can not be negative, which implies > > that then the modulo use case becomes the exception. > > > > Related to this I have the following question: In the > > bug you refer to above, an unsigned variable was used > > for something that is not meant for modulo arithmetic. > > Is this done in the kernel to save space? I.e. because > > 127 would not be enough as maximum but going to i16 takes > > to much space? or is this for compatibility with some > > on-the-wire protocol? > > > > Would an attribute for variables help that tells the > > compiler that if stores to the variable wrap around > > then this is not intended and this is an error? > > > > u8 x [[nowrap]]; > > > > x = 256; // can be diagnosed ? > > There's a problem with representing this, it's a long-standing thing standing > in the way of handling -fwrapv in the IL. Maybe C wants to have > 'nonnegative int' in addition to 'unsigned int', or 'bit int' for the > "just some two-complement bits". > > For GCC we think of -fwrapv to perform signed integer arithmetic as if > promoted to a large enough type first and then truncated to the original > type in the implementation defined manner (modulo reducing). But we > end up doing what the actual hardware does (I'm for example unsure > how that handles INT_MIN / -1 - x86 documents it as raising #DE > which it indeed does, even with -fwrapv). > > volatile int a, b; > int main () > { > a = -__INT_MAX__ - 1; > b = -1; > int z = a / b; > __builtin_printf ("%d\n", z); > } > > raises SIGFPE with and without -fwrapv. I'll note that we document > -fwrapv to only affect addition, subtraction and multiplication > (it also affects negation), One small addition, it also affects the function `abs` (and the corresponding builtins); the same way as negation.
Thanks, Andrew > matching the libgcc *v functions we have > for -ftrapv. -fsanitize=undefined instruments division but > -fsanitize=recover doesn't help. Maybe there's a bug about this > recorded. The documentation of -fwrapv should likely be clarified. > > For recent work I failed to spot the C standards language on > singed overflow behavior for negation, addition, subtraction > and multiplication. I can find it for division though. Can someone > point me to where that is (moved?)? > > Richard. > > > > > > > > > Martin > > > > > > > > > > > >
